Missing Opportunities for Polymorphism

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Polymorphism is one of the grand ideas that is fundamental to OO. The word, taken from Greek, means many (poly) forms (morph). In the context of programming polymorphism refers to many forms of a particular class of objects or method. But polymorphism isn't simply about alternate implementations. Used carefully, polymorphism creates tiny localized execution contexts that let us work without the need for verbose if-then-else blocks. Being in a context allows us to do the right thing directly, whereas being outside of that context forces us to reconstruct it so that we can then do the right thing. With careful use of alternate implementations, we can capture context that can help us produce less code that is more readable. This is best demonstrated with some code, such as the following (unrealistically) simple shopping cart:

The parameter ??? isn't some new fancy Java syntax, it's an indication that we've got a problem. Should I email or snail-mail the item? By the time I get to this point in the code, I've lost the context I need to resolve this question. We could capture method of shipment in a boolean or enum but then we'd need and if-then-else to reconstruct our understanding of which form of ship needs to be called. Another solution would be create two classes that both extend Item. Lets call these DownloadableItem and SurfaceItem. Now let's write some code. I'll promote Item to be an interface that supports a single method, ship. To ship the contents of the cart, we will call item.ship(shipper). Classes DownloadableItem and SurfaceItem will both implement ship.

Each item knows if it can be downloaded or if a regular shipment is required. There is no need to reconstruct lost context. This information has been captured at the time of order. In this example, DownloadableItem and SurfaceItem provide a micro execution context that lets us get on with it.

The code above is representative of the command and the double dispatch patterns. Effective use of both of these pattern rely on careful use of polymorphism. This is why they often play well together. What is noticeably missing are if-then-else blocks.

While there are cases where it's much more practical to use if-then-else instead of polymorphism, it is more often the case that a more polymorphic coding style will yield a smaller, more readable and less fragile code base. The number of missed opportunities lies in the number of such if-then-else statements we can find in our code.